1. Field of the Invention
The present invention relates to the on-line measurement of a two-phase flow or a three-phase flow which does not require the physical separation of the flow of gas from the flow of liquid.
2. Description of the Related Art
The measurement of two-phase flow (gas and liquid flowing together) is very difficult as conventional flow meters cannot handle both gas and liquid simultaneously. Conventional flow meters are divided into three categories: Differential pressure type, volumetric type and mass flow type. The first category comprises orifice plates, venturi tubes, pitot tubes, etc. The second category comprises turbines, positive displacement, magnetic flow meters, etc. While in the third category one finds vibrating type (corollis), heat transfer, momentum type etc.
The first category yields differential pressure output, which is related to the velocity of flow at the downstream pressure tap point, according to Bernoulli's equation EQU .DELTA.p=.rho.v.sup.2 /2g (1)
where
.DELTA.p=differential pressure across the device, PA1 .rho.=density of the fluid at the downstream pressure tap, PA1 v=velocity of the fluid at the downstream pressure tap, PA1 g=gravitational constant. PA1 v.tau.=the terminal velocity of the bubble, PA1 k=Dimensional Constant, PA1 .rho..sub.L =Density of the liquid, PA1 .rho..sub.g =Density of the gas, PA1 .mu.=Viscosity of the liquid, PA1 d=Diameter of the bubble.
Thus a change in the gas/liquid ratio will affect the density .rho. and will appear as a change in velocity. As it is important to know how much gas and how much liquid flow through the meter, this type of flow meter cannot give the answer.
The instruments of the second category cannot distinguish between gas and liquid, and give nearly equal flow rate to both. Again they are unable to determine how much gas and how much liquid pass through them if the flow consists of a two-phase fluid.
The third category suffers from the same disadvantage, plus the added inability of the vibrating type mass flow meters to vibrate properly when the gas content in the fluid exceeds 20% by volume.
As none of the above techniques can resolve the fluid's components, it was required in the past to physically separate them, and then measure each component separately. Two types of separators are known. The old two-phase separator, and a new dynamic separator. The first one slows the incoming fluid, allows the gas to expand, and measures the gas outlet from the top and the liquid outlet from the bottom. If the fluid is made of three components, such as oil and water and gas, a further analysis of the discharged liquid is performed. In the second case, two inclined tubes are used. The gas tends to collect in the upper one, while the liquid flows in the bottom one. Again, in the case of a three-phase fluid, further analysis of the oil/water content is required.
The big draw back of these methods is their size and the delay that they cause in the measurement. In order to allow the small gas bubbles to float up in the liquid, sufficient time must be allowed. The rate at which the bubbles float upwards, is determined by Stoke's Law: EQU v.tau.=k(.rho..sub.L -.rho..sub.g)d.sup.2 /.mu. (2)
where
Thus, in highly viscous liquids, the terminal velocity is very slow and longer time is required for the separation, i.e., the vessel size must increase, and with it the costs.
Most oil wells produce gas, oil and liquid simultaneously. Each component of such a three-phase fluid is required to be measured for fiscal and operation reasons. It is very uneconomical to build multiple separating vessels to measure each well. There is a large economic demand to perform these measurements in the field at low cost. In the case of sub-sea level wells and offshore platforms, size is also at a premium. There is therefore a big demand to measure all three components, without having to physically separate them.